Female Drosophila melanogaster were maintained at five levels of nutrition, with either continuous or intermittent exposure to males. Remating frequency increased with nutrition and was higher with continuous exposure to males. Age-specific and lifetime egg production increased with increasing nutrition, but lifespan peaked at intermediate nutrition. Females on the three highest nutritional levels showed a cost of mating in reduced survival, but only at the highest food level did this reduced lifespan lead to a significant cost of mating for lifetime egg production. The data suggest that remating frequencies in laboratory cultures may evolve to a low enough level for the cost of mating to be only weakly expressed, if at all. Further data are required to assess the importance of the cost of mating in natural populations, where the evolution of low remating frequencies might be expected to be opposed by other costs.

Female fitness in Drosophila melanogaster: an interaction between the effect of nutrition and of encounter rate with males.

In field and laboratory studies of birds, positive associations between male phenotype and success at obtaining extra-pair copulations or extra-pair fertilizations are often interpreted as providing evidence that females are using extra-pair copulations to obtain indirect benefits for their offspring, either through genes for increased viability, or for a fisherian mating advantage I describe a simple model, in which functional fertility (the success of ejaculates in fertilizing eggs) covaries with male phenotype, which can explain the observed associations equally well Under such a model, females pursue extra-pair copulations as insurance against the functional infertility of their mate, and obtain only direct benefits for themselves in their current reproductive event Several studies of birds suggest that a relation between male phenotype and functional fertility is often likely to exist and that there are many potential causes of functional infertility Non-manipulative field studies are unlikely to produce results which distinguish between the two hypotheses, and I discuss several alternative approaches which may allow their resolution

Male phenotype, fertility, and the pursuit of extra-pair copulations by female birds

Publisher Summary
This chapter reviews the current knowledge of variation in sperm morphology over several levels of biological organization: variation within males (both within and across ejaculates), among males, among populations, and among species, along with prevailing hypotheses addressing the adaptive significance of such variation. With regard to developmental mechanisms, three aspects of the physiology of sperm production serve to limit within-ejaculate variation in sperm phenotypes. First, the location of the testes and numerous aspects of testicular physiology of some taxa are clearly adaptations to maintain a homeostatic developmental environment for sperm. Second, developing spermatids may share cytoplasm. Third, sperm phenotypes are predominantly determined by testicular gene expression and hence the diploid genome of the male. Variation across ejaculates but within males can involve several traits including sperm numbers, overall semen quality and individual sperm quality. A special case of intramale variation in sperm form is found in species with sperm heteromorphism, in which different sperm forms are regularly produced by individuals. Differences among males in sperm morphology may derive from both genetic and environmental influences. Theories of condition-dependence basically posit that fitness-related traits are to a large extent dependent on an organism's underlying condition. Conclusions drawn from studies of sperm diversification between natural populations are reinforced by experimental evolution studies of sperm morphology in laboratory populations, as these studies address the evolvability of sperm traits and the nature of selection underlying sperm diversification. Furthermore, a discussion of evolutionary causes and consequences of sperm diversification, along with suggestions of fruitful areas for future exploration is presented.

Sperm morphological diversity

Multiple mating by females: a perspective from quantitative genetics

Do females promote sperm competition? Data for humans

TRIVERS (1972) proposed that females may choose to mate with males of high sexual competence (the ability to supply sufficient sperm for fertilisation). This hypothesis was tested by allowing ewes in oestrus to choose between four tethered adult rams, two of high and two of low semen quality. Contrary to the hypothesis, ewes did not prefer rams of higher semen quality and, even though frequent mating can depress the fertilising ability of ram semen, the attractiveness of each ram to oestrous ewes was not lowered by frequent ejaculation. Also, in contrast to TRIVERS' suggestion, semen quality and male courtship vigour were not consistently related either between or within individual rams. Ewes mated repeatedly during oestrus, receiving nearly six ejaculates each on average, and over two thirds of ewes mated with more than one ram. Repeated mating, rather than a preference for sexually competent males, might function to ensure fertilisation in female sheep and perhaps in other ruminants.

R.M. Gibson

Papers

Semen Quality, Female Choice and Multiple Mating in Domestic Sheep: a Test of Trivers' Sexual Competence Hypothesis